[Fedora-music-list] Low Latency vs. Real Time Kernel - actual latencies ?
Be
be.0 at gmx.com
Sun Apr 5 18:12:14 UTC 2015
Here's a diff of between the configs of Ubuntu's generic and lowlatency
linux-image (amd64) packages:
$ diff config-generic config-lowlatency
3c3
< # Linux/x86_64 3.19.0-12-generic Kernel Configuration
---
> # Linux/x86_64 3.19.0-12-lowlatency Kernel Configuration
69c69
< CONFIG_VERSION_SIGNATURE="Ubuntu 3.19.0-12.12-generic 3.19.3"
---
> CONFIG_VERSION_SIGNATURE="Ubuntu 3.19.0-12.12-lowlatency 3.19.3"
96c96
< # CONFIG_IRQ_FORCED_THREADING_DEFAULT is not set
---
> CONFIG_IRQ_FORCED_THREADING_DEFAULT=y
135c135
< CONFIG_TREE_RCU=y
---
> CONFIG_PREEMPT_RCU=y
145a146
> # CONFIG_RCU_BOOST is not set
251d251
< CONFIG_OPTPROBES=y
381,385d380
< CONFIG_INLINE_SPIN_UNLOCK_IRQ=y
< CONFIG_INLINE_READ_UNLOCK=y
< CONFIG_INLINE_READ_UNLOCK_IRQ=y
< CONFIG_INLINE_WRITE_UNLOCK=y
< CONFIG_INLINE_WRITE_UNLOCK_IRQ=y
457,458c452,454
< CONFIG_PREEMPT_VOLUNTARY=y
< # CONFIG_PREEMPT is not set
---
> # CONFIG_PREEMPT_VOLUNTARY is not set
> CONFIG_PREEMPT=y
> CONFIG_PREEMPT_COUNT=y
565c561
< CONFIG_HZ_250=y
---
> # CONFIG_HZ_250 is not
set
567,568c563,564
< # CONFIG_HZ_1000 is not
set
< CONFIG_HZ=250
---
> CONFIG_HZ_1000=y
> CONFIG_HZ=1000
4998d4993
< CONFIG_DRM_I810=m
7487a7483
> # CONFIG_DEBUG_PREEMPT is not set
7551a7548
> # CONFIG_PREEMPT_TRACER is not set
So it looks like the big differences with the lowlatency config are
enabling CONFIG_PREMPT, CONFIG_IRQ_FORCED_THREADING_DEFAULT, and
CONFIG_PREEMPT_RCU as well as setting CONFIG_HZ to 1000. Would using
these settings in a Fedora kernel have the stability concerns as the RT
patch set? Would there be any other drawbacks?
On 04/05/2015 12:21 PM, Be wrote:
> Has anyone looked into what Ubuntu Studio is doing with the lowlatency
> kernel? Would it be feasible to include a similarly configured kernel in
> Fedora?
>
> On 11/13/2014 10:59 AM, Brian Monroe wrote:
>> I think so too, thanks for chiming in.
>>
>> I'm still waiting to get into the packagers group, but I have a koji
>> account and theoretically could compile an rt kernel. I think the
>> standard naming schema in other distros is kernel-rt. It should be
>> only adding a few lines to the spec file to enable the rt kernel, but
>> when you look at how many kernel update there are for Fedora every
>> week, I'm not sure as to how up to date we'll be able to keep up due
>> to the work load. We're already are down on developers, and people
>> like Brandon are keeping us afloat.
>>
>> Are we going to be ok as a project to be behind a week or two in
>> Kernel releases? Personally I'm for more stable kernels when it comes
>> to music production vs. having the latest and greatest, but I also
>> think that should be a clearly indicated as a feature
>>
>> That being said, I feel strongly as though others should take this
>> task on, if not me, then someone else or better yet, a few of us.
>>
>>
>> I'm looking into the Ubuntu Studio and turns out they dropped the RT
>> kernel as default. They're using a "lowlatency" kernel instead of a rt
>> kernel (though they do still supply an rt kernel but not by default).
>> I do know that users are able to get 1.5 ms latency with zero xruns so
>> I'm guessing they're doing something other than real-time scheduling,
>> I just don't know what. Thoughts?
>>
>> On Wed Nov 12 2014 at 10:40:44 AM Be Ing <be.0 at gmx.com
>> <mailto:be.0 at gmx.com>> wrote:
>>
>> Hello Fedora musicians, I've been lurking this list for a little
>> bit and this is my first time chiming in on something.
>>
>> I think it is important to pursue an official realtime kernel for
>> Fedora. I think a distribution focused on audio without a realtime
>> kernel would have a serious bug, that IMO, would be worth delaying
>> publication for.
>>
>> >So I had a beer with hansomepirate(jdulaney), who is, or was on
>> the kernel
>> sig, last night and we got to talking about a RT kernel.
>> >
>> >Last time we talked to the kernel folks about an rt kernel, they
>> weren't
>> impressed with the "need" for Fedora, but that was before the Spin was
>> officially out.
>> >
>> >Now might be a good time to raise this issue again? I dug through my
>> archives and found this thread. Now that we have an actual spin
>> that's out,
>> we can actually redo some of the testing to have more realistic tests.
>> (multitrack with effects)
>> >
>> >I feel like right now, it's one of the few benefits that the
>> ubuntu studio
>> folks have (or at least claim to have) over us. The other is some
>> semi-proprietary software that on... you know what, never mind
>> it's getting
>> off topic.
>> >
>> >Anyways, does the list think this is worth pursuing?
>> >
>> >>On Wed Feb 22 2012 at 9:10:29 PM Brian Monroe <briancmonroe at
>> gmail.com
>> <http://gmail.com>[https://admin.fedoraproject.org/mailman/listinfo/music]>
>> wrote:
>> >>
>> >> Ok, I redid all the tests, while the system was only running my
>> DE and the
>> >> test, and then again when I put it under duress by running a
>> script that
>> >> looped "du -h /" and "ls -Ral /usr/" over and over. I ran the
>> script twice
>> >> to get my proc up a bit to emulate running some intese delays
>> and reverbs
>> >> or other effects.
>> >>
>> >> Ironically the kernels typically did better when the scripts
>> were running.
>> >> Personally I think there's a clear advantage with CCRMA's
>> kernel or even
>> >> just a preempt kernel in the max lat areas. Those max numbers
>> jumped up
>> >> close to where they were near the beggining of the test if
>> anyone was
>> >> wondering.
>> >>
>> >> Here's the file with both sets of tests and the uname -a info
>> as requested
>> >> by Fernando.
>> >> -Brian
>> >>
>> >>> On Wed, Feb 22, 2012 at 6:54 AM, Brian Monroe <briancmonroe at
>> gmail.com
>> <http://gmail.com>[https://admin.fedoraproject.org/mailman/listinfo/music]
>> >>> wrote:
>> >>> I'll be sure to include that on the next batch. I used the
>> kernel you
>> >>> after installing the CCRMA repo when you use yum install
>> kernel-rt, which
>> >>> happens to be 3.0.17-1.rt33.1.fc16.ccrma.x86_64.rt. I'll go
>> back and
>> >>> include the other info to the old results when I do the load
>> testing
>> >>> tonight or tomorrow.
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>>
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